In this example, the current VDI space is 25GB and we want to increase to 30GB – additional 5GB (The size will be in MB, 1024 x 30 = 30720). Execute the following command,

VBoxManage.exe modifyhd "C:\ITsiti\VM\Windows 7.vdi" --resize 30720

The output from the command,

0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

You can now start up the Virtual Machine. Once connected, open the Disk Management (diskmgmt.msc). If you notice, the new space is still not added and you will see the additional 5GB (Unallocated). So, to merge the additional 5GB, right on C: drive and select Extend Volume.